There exists a need for systems capable of detecting two-phase flow in the reactor coolant loop and for systems capable of detecting liquid level in the core and auxiliary vessels and piping of a nuclear reactor. United States governmental regulations specify "thermal hydraulic" measurement capabilities in all operating power reactors. These needs and requirements are the result of experience gained from a small break loss of coolant accident.
During such an "event" in a pressurized water reactor, there might be a period during which the reactor pressurizer will overfill with water while the primary pumps are operational. The reactor coolant will then remain in two-phase flow through the coolant loop and might have a significant void fraction. Because there is no instrumentation to indicate this condition, the reactor operator is unable to determine that a void fraction exists. As the pumps eventually begin to cavitate and are shut off, the void fraction in the coolant will separate, exposing a portion of the core. Because of the unavailability of liquid level monitoring systems for this contingency, the operators of the reactor might be unaware of these conditions until serious damage results to the core itself.
In a boiling water reactor the coolant in the core is in continuous two-phase flow in normal operation. There is presently no direct reading device which will indicate to the operator what the void fraction is at any particular elevation in the core. This is also true of liquid level readout.
The present system is designed to provide continuous monitoring of coolant conditions at one or more locations within an operating reactor vessel as well as peripheral piping and systems. It can be used to determine the coolant level at a particular location. It is also capable of measuring the void fraction or ratio of the gaseous phase to the liquid phase in the coolant at one or more monitored locations. In addition to these measurements the device will provide a discrete output which can be directly related to the coolant conditions from reactor startup to a finite point in fluid temperature and pressure where the sensor output assumes a different slope. From this point to the point where the fluid temperature and pressure reach "saturation" we choose to define as determined by the orifice sizing in the sensor design. Void Fraction measurement then is represented by the instrument output from this latter point by a nearly linear drop in output reading to a third reading which represent 100% void fraction or a liquid level. This third point in the output reading is also determined by the sizing of the orifices in the instrument, assuming that the driving force "Pressure" is not a limiting factor.